Preparation and Use of (γ,γ-Dioxyallyl)boronates

Abstract

A copper-catalyzed stereoselective 1,4-acylboration of α,β-unsaturated esters with B₂pin₂ and pivalic anhydride has been developed to afford the corresponding (E)-allylboronates with two distinct oxygenated functionalities at the γ positions, which are difficult to prepare by other means. The chemoselective post functionalizations of Bpin and pivalate moieties in the product are also demonstrated.

Key words

allylboronate - borylation - conjugate addition - copper - stereoselectivity

Publication History

Received: 27 February 2023

Accepted after revision: 09 March 2023

Accepted Manuscript online:
09 March 2023

Article published online:
05 April 2023

• References and Notes

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• 14 Experimental Procedures and Characterization Data Synthesis of 2a (Scheme [4, 0].25 mmol scale) Cu(OAc)₂ (4.5 mg, 0.025 mmol), P(3,5-(CF₃)₂C₆H₃)₃ (33.5 mg, 0.050 mmol), and KOPiv (105.2 mg, 0.75 mmol) were placed in a 20 mL Schlenk tube, which was filled with nitrogen by using the Schlenk technique. CPME (1.0 mL) was then added to the tube, and the suspension was stirred for 15 min at ambient temperature. Bis(pinacolato)diboron (158.7 mg, 0.63 mmol) was then added in one portion, and the resulting solution was stirred at the same temperature. After 5 min, pivalic anhydride (69.8 mg, 0.38 mmol) and methyl (E)-5-phenylpent-2-enoate (1a, 40.5 mg, 0.25 mmol) were added dropwise. The reaction solution was stirred at room temperature for additional 18 h. The resulting mixture was directly filtered through a short pad of neutral alumina and Na₂SO₄. The filtrate was evaporated in vacuo and purified by silica gel column chromatography on neutral silica gel with hexane/ethyl acetate (20/1 → 10/1, v/v) and GPC (CHCl₃) to give 1-methoxy-5-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pent-1-en-1-yl pivalate (2a, 82.5 mg, 0.21 mmol) in 82% yield with >99:1 E/Z ratio. (E)-1-Methoxy-5-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pent-1-en-1-yl pivalate ((E)-2a) 82.5 mg (82%); colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 7.27–7.23 (m, 2 H), 7.21–7.19 (m, 2 H), 7.15 (t, J = 7.1 Hz, 1 H), 4.34 (d, J = 9.9 Hz, 1 H), 3.55 (s, 3 H), 2.74 (ddd, J = 13.6, 10.5, 5.4 Hz, 1 H), 2.59 (ddd, J = 13.6, 10.4, 6.0 Hz, 1 H), 2.09–2.03 (m, 1 H), 1.89–1.80 (m, 1 H), 1.73–1.65 (m, 1 H), 1.28 (s, 9 H), 1.25 (s, 12 H). ¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 176.5, 150.6, 142.9, 128.7, 128.3, 125.6, 98.2, 83.3, 57.1, 39.1, 35.4, 33.4, 27.2, 24.9, 24.8, 20.5 (br). ¹¹B NMR (128 MHz, CDCl₃,): δ = 32.7. HRMS (APCI): m/z [M + H]⁺ calcd for C₂₃H₃₆BO₅: 403.2654; found: 403.2666.
• 15 We preliminarily tested several reported conditions, including simple heating, Sc(OTf)₃ catalyst, and ZnBr₂ catalyst, for the allylation of benzaldehyde with 2a, 6, and 7. However, the allylic boronates just decomposed, and the corresponding homoallylic alcohols were not detected at all. Additional investigations are still necessary.
• 16 We preliminarily tried enantioselective conditions using several chiral phosphine ligands. However, the maximum enantiomeric ratio was 75:25 with a phosphoramidite ligand. See the Supporting Information for more details.